Endotoxemia has been proposed as a significant cause of Sudden infant Death Syndrome (SlDS). We examined postmortem sera from heart samples from 21 SIDS cases and 23 controls. The controls were under one year of age and died suddenly and unexpectedly of infection, abuse, suffocation, blunt injury and fire and smoke inhalation. The endotoxin concentration was determined without knowledge of the clinical status by using a kinetic modification of the chromogenic limulus amoebocyte lysate assay. The SlDS cases has insignificant concentrations of endotoxin in serum whereas some of the controls who experienced blunt injury, abuse, or severe infection exhibited moderately elevated endotoxin concentrations. Postmortem interval and postmortem blood culture results did not materially affect endotoxin concentration. Thus, endotoxemia is not a substantial pathophysiologic event in SIDS. We investigated the effects of a murine monoclonal antibody directed against the canine leucocyte CD1 1118 adhesion complex in a canine model of septic shock. The results of the study showed an increase in the neutrophil count but increased concentrations of serum endotoxin and a deterioration of cardiovascular function. Thus, these data suggest that in septic shock, antibody directed against this leucocyte membrane protein complex may be harmful, possibly by impairment of normal leucocyte function.